One more hint, have in this year already read about 10 broken selector shift valves, mainly on 5HP18.Replacement Selector Shift Valve: 1056 327 173 (ZF Part number). The selector valve inside the valve body was originally made out of a kind of plastic/resin, that breaks on many transmissions, so you have the feeling first that shifting is hard and then you can shift and shift and nothing happens actually inside the transmission, this part is now made of a new kind of reinforced 'plastic/metal' material, also when ATF gets hot, the old material expands and shifting gets harder.
example Selector Valve http://www.thectsc.com/products/sele...alve-84-3.html
So basically the same problem as with the small plastic balls in the valve body which get smaller and smaller and finally disappear.
cases I know of within last month, 1 x on an E32 730 M60B30 with 5HP18, another one on an E36 M3 with 5HP18, and one case on an E34 with a 5HP18 and another one on a E36 with 5HP18.
That part is included in every valve body overhault kit, position 50.020 Valve Body Kit 5HP18
To replace that part you have to remove the transmission oil pan, the trans filter, then unbolt the valve body and drop it a bit and then you can change that part. No need to disassemble the valve body, but of you want to do so, here we have some pics when we do that in my garage for valve body overhaul Deep In E32 http://e32b12.blogspot.com/search/label/5HP18

Nice, Frank87- thanks you for updating and clarifying the verified causes, symptoms. Not easy to do but once you get into ATs they become a challenge one can't set aside. You've done well, and not left anyone out by sharing a lot of insights.

Additional info that I got from a specialist is that you actually shouldnt mess with the valvebody except for 2 pieces:
- The selector (broken as shown in shoguns pic), updated design available
- Main pressure regulator (long thick piston in main block) -> Replace it with 5HP19 MPR

The guy said the 5HP19 regulator fits directly and seals better. This means the body holds its pressures better. I believe him because looking at the pictures, you see it is very similar.

Other than that. After all the work I did on 5HP18 boxes hobbywise, I can really say that next time, I will immediately buy a refurbished body from ZF or a specialist and I wont mess with it myself anymore.

I did real good and my box shifts real good but all that work and sourcing all those parts, it's just not worth it.